This invention relates to an imaging system, more particularly to an ultrasonic imaging system. The invention has preferred application in the field of medical diagnosis and treatment. The invention also relates to an associated method.
An ideal medical imaging device would be compact, inexpensive, non-invasive, high resolution, and easily controlled by a physician in a general hospital, office or field environment. The ideal imaging device would enable a diagnostician or treating physician to in effect perform non-invasive dissection, selectively imaging internal structures, and perform minimally invasive or laparoscopic surgery with a facility and clarity as if overlying tissue had been removed.
While substantial advances have been made in recent decades over the traditional techniques of x-ray photography, existing medical image devices still fall far short of the ideal device on one or more criteria. Nuclear Magnetic Resonance and Computer Aided (X-ray) Tomography (MRI and CAT Scanners) offer high resolution and selective viewing of deeply imbedded structures, but neither technique can be reasonably described as xe2x80x9cinexpensivexe2x80x9d, nor the associated devices as As xe2x80x9ccompactxe2x80x9d. Indeed, these devices, requiring specialized facilities and specially trained technicians for their operation as well as heavy capital investment, account for a substantial segment of the burgeoning cost of medical testing. Rather than being available for use as a tool by generalists or in a bedside or office environment, MRI and CAT scanning devices require specialists working in a special facility. The physical bulk of these machines and their monopolization of bedside real estate makes use in the operating theater impractical for the foreseeable future as well as posing logistical problems for field use, even for organizations with deep pockets. The expense of these machines limits their routine application to patients of the world""s richest nations, leaving much of the world""s population under served by late twentieth century medicine.
Ultrasonic imaging, relying neither on intense magnetic fields nor on penetrating ionizing radiation but instead on readily generated acoustic fields, holds greater promise as a vehicle for a portable and less resource-intensive diagnostic tool for the bulk of the world""s population. The potential market for practical devices of this type is vast. Long before the resources will exist to put an MRI machine in every garage, a high-resolution ultrasonic imaging device could be placed in every doctor""s office in every town, easing the unserved bulk of the world""s population into care in accordance with twenty-first century medical standards. To date, ultrasound has not realized this promise. Images are relatively low-resolution, and tomographic; i.e., presenting a single slice of a target at a time. Existing devices are relatively simple in conception, displaying output on a CRT screen as a function of time and direction of return in a single azimuth from out going active pulses, and fall short of the promise of producing easily interpretable images of three dimensional structures in real time. It is desirable to produce acoustic imaging devices capable of greater spatial resolution and higher visual realism. xe2x80x9cVisual realismxe2x80x9d is a measure of the faithfulness to images perceivable if an observer were able to see directly inside a selectively transparent patient, realizing the fantasy of xe2x80x9cx-ray visionxe2x80x9d; the goal of visual realism entails high resolution, low distortion, and correct perspective. Operation of an ideal medical imaging device should also be user friendly. xe2x80x9cUser friendlinessxe2x80x9d emphasizes minimization of special interpretational and operational skills necessary to understand and manipulate device output. User friendliness encompasses intuitive control responsiveness, -including the ability to easily modify device output to focus on structural features of interest. User friendliness and visual realism may be collectively referred to as xe2x80x9cperceptual acuityxe2x80x9d, which encompasses the ability not only to perceive but to readily manipulate high resolution images of complex structures, as if the structures were accessible to direct sight and manual manipulation. The objective is to build a medical imaging device of high perceptual acuity that is also compact, and at minimal cost.
To effectively reconstruct a three-dimensional image from a static array of acoustic sensors, the array must extend in two spatial dimension. Generally, the greater the resolution desired, the larger the array of sensors required. Higher resolution demands larger arrays. However, if a sufficiently large array of sensors is disposed in a rigid mounting the sensors will necessarily not conform to a particular human body surface: employing a filly rigid array in direct contact with a human body limits the array to dimensions over which a soft portion of the body is deformable. This dimensional restriction restricts both resolution and imaged tissue volume. Alternatively, to permit utilization of larger rigid arrays, a secondary medium with an acoustic transmissivity similar to that of the human body may be interposed in a volume between the array and a skin surface. The secondary medium becomes, for the purposes of image processing, just another volumetric region in a three-dimensional model. Interposition of a secondary medium between array and patient, however, may adversely affect ease of use of an ultrasound imaging device and in particular use of such a device in minimally invasive surgical procedures, where the volume occupied by the secondary medium must be penetrated by surgical instruments.
Deforming a small portion of a patient or extending the relatively acoustically dense region represented by a human body with a secondary medium effectively brings the patient to the sensors. A solution to some of the difficulties outlined above is to bring the sensors to the patient, i.e., to deform an acoustic array to conform to an outer surface of the patient""s body. This approach permits utilization of larger array sizes without use of a secondary acoustic medium. Further difficulties are introduced, however.
To reconstruct an image from data collected via an array of acoustic sensors, it is necessary to know the geometric relation or configuration of the sensors; to reconstruct a precise and undistorted image, it is necessary to know sensor positions with precision. Furthermore, since the sensors are brought into contact with a living body which may further be undergoing a medical procedure it is necessary to measure geometric relations between sensors continuously and in real time, particularly if the imaging device is to be used to monitor an ongoing medical procedure.
It is difficult to simultaneously solve for transducer position and target structure utilizing only data received at sensors or transducers via transmission though a target region. Therefore, in order for signals associated with respective transducers to effectively cooperate in construction of an image or three-dimensional model in a system making use of transducers capable of relative movement, it is advantageous to provide an independent means of determining relative transducer positions.
Beyond transducer movement further sources of variation are present in any complex electromechanical system, and an acoustic medical imaging device is no exception. Transducers or other components may require replacement in the course of service, with original and replacement parts of only nominally identical electrical characteristics. Wiring may be replaced or reconfigured, and characteristic values of electrical components may drift with time. Therefore, in addition to having a method of determining the instantaneous configuration of an array of acoustic transducers, it is desirable to provide a method of detecting and compensating for random variations an drift in device characteristics.
A further question to be addressed in development of precise ultrasonic diagnostic tools is the form of visual and other device outputs, particularly with regard to optimizing visual realism.
In summary, difficulties to be overcome in improvement of the current ultrasonic medical imaging art include:
(i) Employment of larger arrays of acoustic sensors than currently employed, with resultant increase in image resolution and visual realism. In particular, finding a method of determining instantaneous relative positions of a deformable array of acoustic sensors in order to utilize data from such an array in image formation.
(ii) Compensating for variation and drift in components of an acoustic imaging system and an associated sensor position determination system.
(iii) Creating a user friendly display and control system of high visual realism.
An object of the invention is to provide a sonic imaging device suitable for forming images of internal structures of a human body.
A further object of the invention is to provide a sonic imaging device which is compact, portable, and easy to use.
Yet a further object of the invention is to provide a sonic imaging device which operates in real time.
Still a further object of the invention is to provide a sonic imaging device whose operation is useful during the execution of further diagnostic and therapeutic procedures.
A more particular object of the present invention is to provide a sonic imaging device with maximal visual realism and user friendliness.
A further object of the invention is to provide a medical imaging device which produces images of higher resolution than existing devices of similar size.
Yet another object of the invention is to provide a medical imaging device which is economical of manufacture in comparison to existing devices of similar resolving power.
Still a further object of the invention is to provide a method for maintaining a device meeting the other objectives in a condition of maximum accuracy.
These and other objects of the present invention will be apparent from the drawings and descriptions herein.
The present invention is directed to an imaging system, particularly useful in medical diagnoses and treatment, which utilizes mechanical pressure waves to obtain data pertaining to internal tissue and organ structures. More particularly, the present invention is directed in part to such imaging systems with (a) position determination subsystems for determining relative positions of electromechanical transducers in real time (b) hardware and associated techniques for calibrating those position determination subsystems and (c) display methods and devices for maximizing an availability of useful information to a user or operator.
In accordance with the present invention, a separate functionality or sub-system is provided for determining relative positions and orientations of the transducers to allow a unique image reconstruction. For a device monitoring a moving target operating in xe2x80x9creal timexe2x80x9d, producing an output with a sufficiently high refresh rate and short enough lag time to simulate continuous current information to a human observer, inter-transducer geometry should be monitored with at least this good a refresh rate and lag.
It is necessary and desirable for maintenance of accurate and precise image reconstruction to have a convenient method of effecting an overall calibration of an acoustic medical image device. Ideally a calibration method should be simple and require minimal additional equipment.
An ideal form of the visual output for many purposes, and in particular for the purpose of ancillary use during a further diagnostic or therapeutic procedure, is one which interposes the visual output in a line of vision of the physician between physician and patient, in such manner that simulated images of organs and other internal structure of a patient detected via ultrasound appear perceptually in spatial locations identical to real locations of these structures with respect to an observer; as if the structures were directly visible to the unaided eye. In short, an ideal ultrasonic imaging device makes the patient appear partially transparent.
An ultrasonic medical imaging apparatus comprises a calibration system, a transducer position determination system and an image formation system. The calibration system adjusts the remaining two systems and is part of the overall medical imaging apparatus. The position determination system discretely determines positions of sensors or sensor arrays considered as rigid bodies, or continuously determines a shape of a flexible essentially two-dimensional body or mat in which the sensors or sensor arrays are embedded. The position determination system may be internal to a mechanical skeleton or armature joining the transducers or transducer arrays, internal to a two-dimensional body in which the transducers or arrays are embedded, or external to such structures. The image formation system may comprise a flat video screen interposed between a subject and an observer or a flexible video screen conforming to an outer surface of a subject. The screens are employed optionally in conjunction with a pair of goggles utilizing synchronized stereoscopic shutters. Alternatively a self-contained pair of stereoscopic imaging goggles may be utilized. In some particular embodiments of the present invention, the image formation system also comprises means to determine a position of a screen with respect to a subject and an observer where a screen is utilized, and an observer and a subject where a screen is not utilized. A common feature of imaging systems in accordance with the present invention is an ability to simulate a direct natural viewing of internal features of a subject.
Outputs of the position determination system along with transducer signal outputs serve as inputs of a computational stage of the image formation system. In one embodiment of an imaging apparatus in accordance with the present invention, a plurality of acoustic transducers are disposed in rigidly mounted subarrays, each subarray containing at least one transducer. The transducers of each subarray are respectively maintained in an effectively fixed geometric relationship by a rigid tile or mounting, while individual tiles are mounted on a common flexible substrate, by mechanical linkages, or else are completely structurally independent. Rigid mounting in general entails from zero to six rotational and translational degrees of freedom of each transducer subarray with respect to each adjacent subarray. Initial calibration of the ultrasonic medical imaging apparatus is achieved by placing the tiles or transducer subarrays in known locations around a calibrating body of pre-determined structure. In one embodiment the calibrating body takes the form of a fluid filled cylinder with an internal target such as a sphere immersed in the cylinder at a pre-determined location. The position determination system is adjusted to return the known locations of tiles or transducer subarrays around the calibrating body. This adjustment is accomplished effectively by iterative setting of a plurality of correction coefficients to xe2x80x9czero outxe2x80x9d position determination error, or xe2x80x9cdial inxe2x80x9d a correct set of positions. An effective number of compensatory adjustments or correction coefficients are also provided in association with the image formation system to dial in an accurate and undistorted representation of the cylinder with an image of the internal target at the pre-determined location. Drifting or uncertain parameters and in particular varying electrical component characteristics and timing uncertainties are thereby effectively compensated without individual determination of the parameters, a process similar to adjustment of a CRT image via a small number of controls.
Calibration ideally is undertaken according to a schedule based on a planned maintenance system in accordance with well-known preventative and corrective maintenance principles; comprising a periodic schedule, a use-based schedule, and an ad-hoc basis responding to replacement of failed system components. Calibration compensates for variations both in the image formation system and the position determination sub-system.
A position determination system for an array of tiles mounted to a flexible substrate may be internal and/or external, or a combination thereof xe2x80x9cInternalxe2x80x9d means that the position determination system is substantially contained in or directly attached to a flexible tile substrate or web to which transducers or acoustic tiles are affixed, while an xe2x80x9cexternalxe2x80x9d system is one in which components not directly mounted to a flexible substrate or coupling of the tiles must cooperate in order to collect sufficient information to determine tile position. If the tiles are filly independent, not subject to any structural constraints in their positioning, then only an external position determination system is possible. One may also subdivide position determination systems into discrete and continuous systems. A xe2x80x9cdiscretexe2x80x9d system directly determines positions and orientations of the tiles considered essentially as a lattice of rigid bodies, the shape of any intervening surface being irrelevant. A xe2x80x9ccontinuousxe2x80x9d system monitors the shape of a flexible substrate or web or possibly a patient at a dense array of points, tile position being secondarily determined via attachments to the substrate. A continuous system is based on a mathematical model of a continuous surface, a discrete system on a model incorporating a discrete array of objects.
Continuous and discrete position determination systems may be realized either internally or externally. All four variations; internal-discrete, internal-continuous, external-discrete, external-continuous; are considered as specific embodiments of the instant invention with some specific attendant advantages and disadvantages. For many applications an internal position determination system is preferred, an internal system being less restrictive of access to a patient on which transducers are disposed. External systems however may be initially simpler of implementation, and have the potential or allowing completely unfettered placement of transducers, since no web or substrate is necessary, so the transducers or tiles may be positioned in order to maximally acoustically illuminate regions of special interest. Internal position determination systems are less obstructive to a user. Internal systems may make use of digitally encoded mechanical positioners or optical or acoustic ranging signals (discrete systems), or deformation sensitive outputs (continuous systems) involving, for example, piezoelectricity. External position determination systems may make use of acoustical or optical ranging signals or methods for monitoring the shape of a complexly curved surface. For reasons which will become clear in what follows, a preferred embodiment utilizes a continuous internal position determination system.
Between a pair of rigid objects there are in general six degrees of freedom: Three rotational and three translational. In a free-body case, considering each mounting plate or tile as the physicists"" well-known rigid body, with an independent coordinate system affixed thereto, we require six parameters to fully specify the position and orientation of a second rigid body with respect to that coordinate system. However a full six degrees of freedom of motion between adjacent plates or tiles in an acoustic imaging system are not always necessary nor desirable: Distance between the plates may in general be conveniently held fixed, either absolutely or in a arcuate sense in a deformable surface, eliminating two degrees of freedom. Rotation about an axis perpendicular to a major flat surface of the tile is generally of no importance in conformation to a body surface, eliminating a third degree of freedom. Envisaging a row of plates having centers disposed along a line or arc disposed in a flexible substratum, it would in general be advantageous to allow one degree of freedom for depressing or elevating this arc and two for rotating a plate about the centers, although less freedom will suffice for many applications.
Mechanical linkages between adjacent tiles generally have an effect of reducing degrees of positional freedom. For example, a simple hinge between two bodies reduces the degrees of freedom from six to onexe2x80x94the hinge angle. However, the degrees of freedom cannot be mechanically reduced indiscriminately, but must be downsized in some coordinated fashion. If every adjacent pair of tiles were joined by a hinge, for example, the resulting structure would be able to bend only in one of two orthogonal axes at a time, hence unable to conform to a multiply curved surface. A related consideration to the number of mechanically allowed degrees of freedom is choosing parameters to be measured to fix a configuration of an array of rigid bodies. It is not necessary to know six parameters between every pair of bodies for this purpose. In the case that mechanical connections exist reducing the overall freedom of movement between adjacent bodies relative to the free body case, the questions of which degrees of freedom to mechanically eliminate and which degrees of freedom to measure become largely the same question. Illustrating the principle that getting an optimal answer often depends on asking an optimal question, the present invention answers these questions simply in the case of a partially rigid mechanical frame still possessing sufficient flexibility to wrap around an exterior surface of a patient.
A partially rigid mechanical frame may be combined with a method of directly encoding frame angles as outputs, a so-called digital encoding, thereby determining relative sensor positions. Limitations exist on the angular resolution of such a mechanical system, and accuracy is subject to degradation through accumulating wear. Nonetheless, such a system is conceptually simple and has the potential of being physically robust. A mechanically based system is suitable for applications where the highest attainable positional precision is not a requirement.
In an alternative to a purely mechanical system, a mechanically linked frame has a non-mechanical or optical means of position determination. In particular a mechanical frame is provided with laser interferometric rangefinders. Laser interferometry is advantageously combined with partially rigid frames, since such frames permit distance measurement to be accomplished along lines of sight determined by telescoping frame members instead of requiring tracking of completely independently moving bodies. Variable frame angles may be determined by appropriately chosen distance measurements. A variety of laser interferometric techniques, including use of optical sensor arrays containing integrated chip-level logic for pre-processing of pixel-by-pixel optical data in fringe protection, are discussed in the sequel.
A laser interferometric position determination system is relatively expensive, but has the potential of great accuracy and precision and is desirable where the highest attainable absolute positional precision is a requirement, as for example when acoustic imaging is combined with therapies involving the focusing of destructive energy on tumors. A simple yet useful combination of a partially rigid mechanical frame with determination of a small number of geometric parameters occurs in an embodiment comprising a pair of subarrays separated by a frame encompassing 0, 1 or 2 degrees of freedom. Used in conjunction with a pair of stereoscopic goggles, the frame, pressed against a patient, provides a direct simulation of a close-up binocular view into the patient at a point of contact; parameters like focus plane, depth of focus, and magnification being adjustable by electronic controls, the spacing and angle between adjacent transducer arrays being subject to mechanical adjustment. In case of 0 degrees of freedom, the device comprises a single bi-lobate array affixed to a rigid frame.
Another internal position determination system comprises an array of strain gauges distributed in a flexible substrate or web to which acoustic transducers are also attached. A pointwise determination of curvature of a surface in two orthogonal axes is equivalent to a specification of the configuration or shape of that surface, the accuracy of the specification being a function of the fineness of a grid on which curvature is determined. In practice, for a substrate of substantial rigidity but still sufficiently flexible to allow conformation to an external surface of a patient""s body, position determination via measurement of local average curvature on an achievably fine grid may be satisfactory for many applications. Numerous methods of measuring curvature of a substantially planar substrate via a regular two-dimensional array of devices responsive to local strain will suggest themselves to one skilled in the art. For example, a web of conductive and flexible bimetallic strips whose conductivity varies with elastic strain may be woven together to form a fabric with warp and woof, with relatively high resistance contacts at each junction of perpendicular strips. To conduct a complete strain measurement, a current is passed across each warp strip in sequence, a voltage measurement between each pair of woof strips then determining bending along a corresponding length of that warp strip. After completion of a sequence including all warp strips, currents are then passed across each woof strip in sequence, voltage measurements being taken respectively between each pair of warps strips. A complete scan then repeats. This scheme utilizes one set of woven lines both as sensors and leads. Another scheme might for example use a first set of lines intersecting at right angles as read-outs or lead lines, and a second set of lines composed of differing material as sensors, disposed at a 45xc2x0 bias relative to the first set of lines. One sensor line is conductively affixed during manufacture across each junction of lead lines. Lead lines in this case would be disposed at higher density than sensor lines, at least three being required to read each junction, which higher density might be accomplished by provision of a two layers of lead lines in a first orientation, separated from each other by an insulating layer, and one layer of lead lines in a second orientation. More sophisticated sampling schemes may also be envisaged than simple raster scans described above, reacting to strain rate information to concentrate monitoring in regions of greatest rates of change, or where subsequent measurements would reduce positional uncertainty most efficiently.
External position determination systems rely upon means external to an array of sensors or rigid sensor-carrying tiles to determine the configuration or relative positions thereof. Such systems include external transmitters, either optical, acoustic, or radiometric, which interrogate or track targets embedded on a blanket or substrate, other (substrate) passive systems, such as projecting a pattern on the substrate for computer vision interpretation, or projection of signals from the substrate aiding in tracking of points thereon. Because external position determination systems share airspace above a patient with attending physicians, imaging device displays, and other equipment, the issue arises of access and interference. Two solutions are disclosed in accordance with the present invention: a stand-off frame and an image-freezing system.
In case of a stand-off frame, a flat screen providing high-quality acoustic images is disposed at a small distance from a surface of a patient, for example, an abdominal surface, the screen being mounted on a movable arm, or space frame partially enclosing the patient. Position determination means are disposed to utilize a gap between the screen and the patient, so as not to interfere with observation of internal structures of the patient on the screen. Laparoscopic instruments and other devices may be inserted into or attached to the patient behind and or laterally of the screen. Position determination means may take the form of optical or acoustic ranging devices. Alternatively, in another feature of the present invention, a grid is projected onto on a surface of a flexible substrate or a skin surface of the patient by, for example, laser scanning. One or more digital photochips records or charge coupled devices (CCD""s) record an image of this grid, computer programs known in the art calculating a three-dimensional shape of a deformed two-dimensional surface by means of two dimensional shapes of laser scanned lines recorded on the digital photochip(s). The latter method may also be utilized in the case of a flexible video-screen disposed directly on the patient, as disclosed in published International Application No. PCT/US9,808,177, publication No. WO98/47,428. In this case a laser scanner and CCD are mounted in a position above the patient so as to not interfere with operating personnel in a normal range of operating positions. In the event of moderate loss of a digital image of laser scanned lines, a processing system establishes a current position of the surface by extrapolation. In the event of severe image loss, an alarm will sound, alerting attending personnel to stand back from the patient to allow recovery of the digital image. Alternatively, if an attendant wishes to deliberately bend over the patient he or she may push a xe2x80x9cfreezexe2x80x9d button, which will digitally freeze the acoustically derived image on the video screen(s). A message or other indication on the video screen and/or a special tone may indicate to attending personnel that the image is now frozen. In accordance with another feature of the present invention discussed below, an observer bending over the patient, or in general changing his or her position with respect to the patient, may expect to see a faithful image of selected internal features of the patient with respect to his current viewpoint.